1. Field of the Invention
This invention relates to photovoltaic devices and more particularly to back contacted metal-insulator-semiconductor (MIS) cells having a non-shadowed illuminated surface.
2. Description of the Prior Art
A photovoltaic cell is a device that directly converts photon radiant energy into electrical energy. In a semiconductor photovoltaic device, photons of sufficient energy react with the semiconductor atoms or molecules to produce negative and positive electrical charge carriers, electrons and holes, which can move freely throughout the semiconductor. The object of the device is to collect electrons at one electrical terminal and holes at a second electrical terminal before they recombine elsewhere within the semiconductor. To cause these free charge carriers to flow to their respective terminals, a barrier region is formed between the two terminals such that electrons move more easily than holes across the barrier in one direction and conversely for holes in the other direction. Until recently the most effective barrier in a semiconductor photovoltaic cell has been found to be a P-N junction, which is formed at the interface between P-type and N-type impurity contained regions of the semiconductor.
Efforts in the last ten years to produce efficient and economically viable terrestrial solar cells have depended in substantial part on being able to form P-N junctions in semiconductor materials at a high rate of manufacture. Recent developments in minority carrier metal-insulator-semiconductor (MIS) cells have demonstrated a barrier region that is of equal or greater effectiveness as the P-N junction and that is manufactured by a process which involves only the application of film coatings to the semiconductor surfaces.
The basic MIS cell structure comprises an approximately 100 micrometer thick P-type silicon substrate with a set of thin film contacts and coatings on both the top and bottom surfaces of the cell. The top or illuminated surface of the cell comprises a relatively thin layer of silicon dioxide, then an aluminum film contact grid or grating, and then an outer film coating of silicon nitride or silicon monoxide which provides a continuous inversion or depletion region across the illuminated surface, serves as an anti-reflection layer, and protects the aluminum grid and the underlying silicon from oxidation or other forms of atmospheric corrosion. The back or dark side of the substrate comprises a thin aluminum ohmic contact covering substantially all of the back surface of the cell. These processing steps are relatively uncomplicated and easy to accomplish. No P-N junction formation is required in the fabrication of this cell. Since relatively low temperature processing is used, high values for minority carrier diffusion length or lifetime can be achieved. This structure may also be used with semicrystalline, polycrystalline, or amorphous silicon; however, this will entail a reduced efficiency below that of monocrystalline silicon due to the shorter minority carrier diffusion lengths and lifetimes encountered in such materials. A shortcoming of this cell is the shadowing of the illumination by the opaque or nearly opaque contact grid metallization on the illuminated surface of the cell. Further, any attempt to increase the current carrying capacity of this MIS contact by increasing the width or number of the metal grid lines will result in increased shadowing.
A design direction that has produced fruitful results for avoiding illuminated-side contact shadowing is to place both P and N doped collector regions on the back or dark side of a photovoltaic cell. A leading design in this technology is the tandem junction cell which has been developed by Texan Instruments and is represented by U.S. Pat. No. 4,133,698, dated Jan. 9, 1979, to Chiang, et al, entitled "Tandem Junction Solar Cell." This cell comprises a P-type substrate with a layer of N-type material emplaced upon the upper or illuminated surface and a series of interleaved parallel P-N junctions on the back or dark side of the cell. The upper N-type layer or the P-type substrate is not electrically contacted and participates only indirectly in the collection of photogenerated carriers by providing an electric field to suppress front surface recombination and to enhance collection at the back side junction. A recent publication points out that if the semiconductor material has the properties of very low surface and bulk carrier recombination, the upper junction or front surface cell offers little or no advantage. However, these properties are not readily obtained by low-cost processes. In either event, since all metallization is on the back side of the cell, shadowing is avoided. The principal shortcoming of this cell is the difficulty in precisely forming the parallel P-N junctions on the dark side of the cell.